The present invention relates to an unsaturated polyester resin capable of forming a cured article having excellent gloss retention and yellowing resistance, a coating material and a gel coating material which use the same, and a fiber reinforced plastic (FRP) molded article using the same on the surface.
Unsaturated polyester resins are used in various FRP molded articles, coating compositions and gel coating materials because of their excellent chemical, physical, mechanical and electrical characteristics such as their wide curing temperature conditions and rapid curing rate. However, conventional unsaturated polyester resins have drawbacks in that not only is the gloss drastically lowered, but also yellowing occurs when exposed outdoors for a long period because of poor weathering resistance. To solve the drawback, Japanese Unexamined Patent Application, First Publication No. Hei 7-157645 suggests an unsaturated polyester resin composition derived from an acid component comprising an alicyclic saturated acid and an aliphatic unsaturated acid, and an alcohol component selected from aliphatic alcohol and alicyclic alcohol, while First Publication No. Hei 9-263692 suggests an unsaturated polyester resin composition derived from an acid component comprising hexahydrophthalic anhydride and an unsaturated dibasic acid and a polyhydric alcohol component. Although the yellowing resistance of these unsaturated polyester resin compositions is improved, the gloss retention is not improved.
EP-A-0400884 suggests an unsaturated polyester resin composition using 2-methyl-1,3-propanediol as an alcohol component. U.S. Pat. No. 6,268,464 suggests an unsaturated polyester resin composition obtained from 30 to 70 mol % of at least two diols, 0.5 to 8 mol % of 2-butyl-2-ethyl-1,3-propanediol, an aromatic carboxylic acid and other two carboxylic acids. In these inventions, only the yellowing resistance is improved, and the gloss retention is poor.
In these prior arts, although the yellowing resistance was improved by an improvement in the composition of the raw materials, gloss retention is insufficient and, thereby causing chalking, and lowering of the gloss within a comparatively short time. Therefore, these prior arts do not satisfy both yellowing resistance and gloss retention which are factors of the weathering resistance.
The regulation of the release of a styrene monomer into the atmosphere has recently become more strict in view of environmental problems; therefore, resins which experience low styrene vaporization and have a low styrene content have been required. As a method of preparing an unsaturated polyester resin which experiences low styrene vaporization and has a low styrene content, a method of reducing the styrene content using a CPD unsaturated polyester having a low molecular weight and a method of adding paraffin wax are known. According to these methods, the weathering resistance cannot be satisfied, although the vaporization amount and content of the styrene monomer can be reduced.
An object of the present invention is to provide an unsaturated polyester resin composition wherein both the gloss retention and the yellowing resistance in a weathering test have been improved, a coating material using the same, and a molded article thereof.
To solve the problems described above, the present inventors have made intensive studies about unsaturated polyester resin compositions and found that an unsaturated polyester resin composition which has excellent gloss retention and yellowing resistance (e.g., a gloss retention of 60% or more and a color difference of 20 or less) and which also has a small content of a polymerizable unsaturated monomer can be obtained when an unsaturated polyester resin composition is prepared such that the xe2x80x9cshrinkage stress upon coolingxe2x80x9d is 17 MPa or less and the ratio of the xe2x80x9cshrinkage stress upon cooling/elastic limitxe2x80x9d is 1 or less in a cured article of the unsaturated polyester resin composition when the temperature is reduced from 70xc2x0 C. to 20xc2x0 C. Thus, the present invention has been completed.
The present invention provides an unsaturated polyester resin composition comprising an unsaturated polyester (A) and a polymerizable unsaturated monomer (B), wherein the xe2x80x9cshrinkage stress upon coolingxe2x80x9d of a cured article thereof is 17 MPa or less and the ratio of xe2x80x9cshrinkage stress upon cooling/elastic limitxe2x80x9d is 1 or less. The present invention also provides a coating material and a gel coating material which use the unsaturated polyester resin composition, and a fiber reinforced plastic molded article.